Motor learning and adaptation: The role of motor abundance

Performance of a functional motor task typically involves more joints than strictly required to achieve the goal, such as positioning the hand at a particular location in space. This leads to the need to specify a particular combination of joints, referred to as the degree-of-freedom problem (Bernstein 1967). However, instead of a problem, the available motor redundancy can be viewed as beneficial because it allows for the task goal to be achieved in a variety of ways, imparting movement flexibility. Thus, redundant degrees of freedom have been referred to as motor abundance to reflect the benefit of having a redundant motor system (Latash et al. 2000). Still, in principle, each trial of performing a task requires a particular combination of joints to be used. A recently proposed control scheme suggests, however, that an active choice of a particular joint combination for a given performance is not a feature of nervous system control. The Uncontrolled Manifold (UCM) hypothesis proposes a control scheme where motor abundance is taken advantage of by the nervous system when controlling functional movement tasks. This hypothesis has recently been substantiated for a variety of motor tasks, the results of which have been replicated in formal model simulations of the theory (Martin et al. 2004; 2005). The UCM hypothesis is that combinations of motor elements that would alter the desired task performance are restricted by this control scheme, while other redundant combinations that do not affect task performance are hypothesized to be relatively less restricted. The method of the UCM allows actual joint or muscle variance to be partitioned into two components (Scholz and Schoner 1999; Schoner 1995). One component is variance that lies within a subspace, or a UCM, that corresponds to motor abundant joint combinations which lead to an equivalent value of a corresponding performance variable (e.g. hand path) across repetitions (VUCM). The other component of variance (VORT) lies in the orthogonal space, or complement of the UCM. This variance represents joint configurations that lead to values of the performance variable different from the "desired" value. The UCM hypothesis predicts that VUCM is typically substantially larger than V ORT. This prediction has been confirmed in many previous studies involving a variety of tasks, including those more postural in nature (Scholz and Schoner 1999; Reisman et al. 2002; Krishnamoorthy et al. 2003; 2004; 2005) to more skilled upper extremity tasks (Scholz et al. 2000; Tseng et al. 2002, 2003; Latash et al. 2001; 2002; 2003). The aim of this dissertation was to test if and how this control law evolves when learning a novel, skilled motor task and whether the use of motor abundance is enhanced when a normal motor system adapts to novel environments. Such knowledge would have implications for understanding the potential limitations on learning and adaptation for individuals who have mechanical restrictions of their degrees of freedom (DOFs) or the need to find strategies to enhance the use of motor abundance in patients with neurological dysfunction to aid in learning and adaptation.; My first study attempted to determine the effects of practicing a novel motor task on the use of motor abundance over the course of learning. The experiment had naive subjects practice throwing a Frisbee at a target from a well-controlled initial position. Joint configuration variance was partitioned with respect to hand/Frisbee movement along a straight path to the target (typically referred to as movement extent in reaching studies), movement orthogonal to the path (movement direction), hand path velocity, and the hand's orientation to the target. With practice, both VUCM and VORT decreased with respect to all of the above performance variables. The decrease in VUCM was smaller than the decrease in VORT, however, when analyzed with respect to the control of movement direction and the hand's orientation to the target...